1. Field of the Invention
The present invention relates to a magnetic recording material such as a video tape, an audio tape, a memory tape, an audio cassette tape, a magnetic disc, a magnetic card, or the like. More particularly, it relates to a magnetic recording material superior to known magnetic recording materials especially in envelope wave form characteristics and further having balanced properties such as video S/N, magnetic print through of audio signal and chroma output, etc. Hereinafter, even though the invention is applicable to all types of magnetic recording medium forms, for simplicity, the description will be given mainly with reference to a helical type VTR tape, since the effect of the present invention is particularly marked for a helical type VTR and further since it is presently, in general, thought that VTR tapes require the highest level of magnetic recording and reproducing techniques.
2. Description of the Prior Art
In general, a magnetic recording material has a structure in which a magnetic recording layer is formed on one surface of a non-magnetic support and, if desired, an electroconductive and lubricating layer is formed on the other surface of the support. However, in most cases, the electroconductive layer or the lubricating layer is omitted. In special cases, magnetic recording layers or lubricating layers can be formed on both surfaces of the support.
Non-magnetic materials are used in most cases as the support, however, in special cases, a magnetic layer is added to prevent magnetic print through.
The magnetic recording layer mainly comprises a ferromagnetic material and a binder.
The ferromagnetic material can be classified by shape as needle-like, granular, plate-like and spindle-shaped, however, a ferromagnetic material with a needle-like or granular shape is mainly used.
The ferromagnetic material can also be classified as follows:
I. IRON OXIDE GROUP FERROMAGNETIC MATERIALS SUCH AS .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, or those iron oxide group materials additionally containing metal atoms such as Co, Ni, Mn or Cr, as disclosed in Japanese Pat. Publication Nos. 5515/1961 (corresponding to U.S. Pat. No. 3,046,158), 6538/1966, 6113/1967, 10994/1973, 15759/1973, 27118/1973 (corresponding to U.S. Pat. No. 3,794,519), U.S. Pat. Nos. 3,117,933, 3,573,983, 3,652,334, 3,671,435 and 3,748,270.
ii. alloy group ferromagnetic materials which mainly contain at least one metal selected from Fe, Co, Ni or a mixture thereof, i.e., Fe, Co, Ni, Fe--Co, Fe--Ni, Co--Ni, Fe--Co--Ni, as main components and additionally containing other metals such as B, Al, P, Sc, V, Cr, Mn, Cu, Zn, Ga, As, Se, Y, Zr, Mo, Ru, Rh, Ag, Sn, Sb, Te, W, Au, Pb, and Bi, etc., as disclosed in Japanese Pat. Publication Nos. 41158/1972 (corresponding to U.S. Pat. No. 3,767,464), 41719/1972 (corresponding to U.S. Pat. No. 3,607,218), 23755/1973 (corresponding to U.S. Pat. No. 3,607,675) and 1997/1974 (corresponding to U.S. Pat. No. 3,726,664), and Japanese Pat. Application (OPI) Nos. 5057/1971 (corresponding to U.S. Pat. No. 3,634,063), 1353/1972 (corresponding to U.S. Pat. No. 3,756,866), 13032/1972 (corresponding to U.S. Pat. No. 3,790,407), 28999/1973 (corresponding to U.S. Pat. Application Serial No. 492,819, filed July 7, 1974), 79153/1973 (corresponding to U.S. Pat. No. 3,748,119), 41899/1974 (corresponding to U.S. Pat. No. 3,837,912), 43604/1974 (corresponding to U.S. Pat. No. 3,865,627) and 99004/1974 (corresponding to U.S. Pat. Application Ser. No. 320,630, filed Jan. 2, 1973).
iii. other ferromagnetic materials such as CrO.sub.2 or those materials additionally containing Te, Sb or other metals (e.g., B, Al, Ca, V, Fe, Cu, Y, Zr, Mo, Sn, W, Pb, etc.), as disclosed in Japanese Pat. Publication Nos. 11617/1967 (corresponding to U.S. Pat. No. 3,278,263), 28366/1969 (corresponding to U.S. Pat. No. 3,449,073), 43437/1973 (corresponding to U.S. Pat. No. 3,819,411), and 41759/1974 (corresponding to U.S. Pat. application Ser. No. 284,003, filed Aug. 28, 1972), and U.S. Pat. Nos. 3,371,043, 3,512,930, 3,574,115, 3,585,141, 3,586,630, 3,647,540, 3,687,726, 3,696,039, 3,736,181 and 3,769,087, etc.
On the other hand, the characteristics required for magnetic recording materials become more sophisticated year after year. Particularly, the requirements for helical type video tapes are severe.
In general, the recording density of a magnetic recording material is about 5 .mu. or higher when calculated in term of recording wavelength. However, the recording density of a helical type video tape ranges from about 1.5 to 3 .mu., that is, the highest technical level possible today is required.
Since a signal of short wavelengths is recorded in a helical type magnetic video tape, the in-put or out-put signal is affected by the degree of roughness on the surface of the video tape used and further the particle sizes and magnetic properties of the ferromagnetic material employed particularly affect the video S/N ratio. Accordingly, it is necessary to decrease the size of the particles in order to improve the video S/N ratio, but the properties of the magnetic print through the audio signal of the video tape become poor.
Heretofore, the following disadvantages have occurred when such video tapes for recording a short wavelength are produced using an iron oxide ferromagnetic material containing cobalt metal atoms.
(1) The envelope wave form of the output signal is distorted.
(2) The balance between the video S/N ratio and the magnetic print through of audio signal ratio is difficult to control.
Particularly, the distortion of the envelope wave form greatly affects the video image quality, and accordingly is undesired for practical use.
Heretofore, improvements in the video S/N ratio have been achieved by the multiplication effect of decreasing the particle size of the ferromagnetic substance, that is, fine graining, and decreasing the degree of the roughness of the surface of magnetic recording layer to improve the sensitivity of the ferromagnetic medium. On the other hand, improvements in chroma output and magnetic print through of audio signal have been achieved by improvements in the characteristics of a ferromagnetic material and improvements in the squareness ratio (Br/Bm) in the magnetization curve (B-H curve) of a magnetic recording tape. However, no techniques or approaches to improve the envelope wave form fluctuation factor have been proposed in the prior art.
The video output wave form preferably always has a constant maximum level of reproducing output when the recording input level is constant as shown in FIG. 1 (the ideal output wave form). However, the actual output fluctuates as shown in FIG. 2 (an actual output wave form) due to non-uniform contact between the magnetic head and the magnetic tape and other reasons.
Although there is no standard method in the art for representing this fluctuation of the output level, it has been defined in the present invention in order to be able to quantitatively evaluate the fluctuation. In the present invention, the envelope wave form fluctuation factor is defined as the ratio of the output fluctuation width to the maximum output level.
In the present invention, evaluations of the envelope wave form were conducted using the following relationship (I): EQU (vo/Vs).times.100=Envelope Wave Form Fluctuation Factor (%) (I)
wherein Vs represents half of the peak-to-peak value of the carrier signal output and Vo represents the width of the fluctuation of the carrier signal output.
According to this evaluation method, fluctuation factors obtained using known technical levels are greater than 15%, however, this factor is preferably less than 13%.
Research on improving the envelope wave form using iron oxide group ferromagnetic materials has now been made and a magnetic recording material has been found which provides an improvement in the envelope wave form but does not degrade the other characteristics such as video S/N ratio, magnetic print through of audio signal ratio and chroma output, etc.